U.S. patent application Ser. No. 09/024,340 filed on: Feb. 17, 1998 by the same inventor, C. Y. Wu of this application disclosed an automatic optical levelling, plumbing, and verticality-determining apparatus having a plumb device secured with a horizontality calibrator universally pendulously mounted on the frame, in which a plurality of screws are provided for operatively adjusting the horizontality calibrator by slidably moving the inner ring of the calibrator about the longitudinal center of the pendulating stem of the plumb device until a gravity center of the plumb device is aligned with the plumb line of the plumb device as checked by a bubble level.
However, the screws used for the adjustment may cause deformation for the instrument elements having thin wall and may also accumulate stress as tightened by the screws during their adjustment, thereby gradually releasing the stress to further cause deviation from the horizontality or verticality, and influencing the measurement precision accordingly.
The present inventor has found the drawbacks of the conventional optical device and invented the present optical calibrating apparatus.